1. Field of Invention
The present invention relates to an optical pick-up device comprising a plurality of laser light sources and serving to carry out recording on an optical recording medium and reproduction of different optical recording media, and more particularly to an optical pick-up device capable of performing recording and reproducing operations with high precision by efficiently utilizing a laser beam emitted from each light source.
2. Related Art
There has been known an optical pick-up device capable of reproducing different optical recording media, for example, a CD-R/RW (Read/Read and Write) together with a CD and a DVD. For example, there has been known an optical pick-up device of a 2-light source type comprising a semiconductor laser having a wavelength of 780 nm together with a semiconductor laser having a wavelength of 650 nm or 670 nm.
In order to constitute such an optical pick-up device to be compact, one of the present applicants has proposed an optical pick-up device having such a structure that an optical system of each laser beam is shared to guide each semiconductor laser toward the optical recording medium side through a common optical system and a return light is also guided from the optical recording medium to a photodetector for information reproduction and servo signal generation through the common optical system Japanese patent Laid open No 10-112056 (Japanese Appl. No. 8-263997).
The optical pick-up device comprises a first semiconductor laser for emitting a first laser beam, a second semiconductor laser for emitting a second laser beam having a different wavelength from that of the first laser beam, an objective lens for causing the first and second laser beams to converge as light spots on an optical recording medium, a common light receiving element for receiving a return light sent from the optical recording medium, and first and second beam splitters, the first laser beam being guided to the objective lens through the first and second beam splitters, the second laser beam being guided to the objective lens through the second beam splitter and the return light being guided to the common light receiving element through the first and second beam splitters. Moreover, the first and second beam splitters have same optical characteristic in which a transmittance is set to 50% and a reflectance is also set to 50% for each laser beam.
However, the optical pick-up device thus constituted has respects to be improved. More specifically, when laser beams having different wavelengths are to be guided to the common light receiving element, the quantity of light is attenuated every time the laser beam having each wavelength is reflected by and transmitted through each beam splitter. Therefore, there is a drawback in that a light quantity loss is great as a whole of each laser beam.
In order to carry out recording on the CD-R/RW (the CD and the CD-R/RW will be generally referred to as CDs), particularly, it is necessary to reduce the light quantity loss of the laser beam for CD recording, thereby forming a light spot in a sufficient quantity of light on a CD recording plane. In the optical pick-up device having the above-mentioned structure, however, half of the light quantity loss is generated by the beam splitter. Therefore, utilization efficiency of the laser beam for recording is low.
In consideration of such a respect, it is an object of the invention to propose an optical pick-up device capable of reducing a light quantity loss by using a common optical system and a common light receiving element, thereby properly reproducing a CD and a DVD and performing record on the CD.
In order to solve the problems, the invention provides an optical pick-up device comprising a first semiconductor laser for emitting a first laser beam, a second semiconductor laser for emitting a second laser beam having a different wavelength from that of the first laser beam, an objective lens for causing the first and second laser beams to converge as light spots on an optical recording medium, a common light receiving element for receiving a return light sent from the optical recording medium, and first and second beam splitters, the first laser beam being guided to the objective lens through the first and second beam splitters, the second laser beam being guided to the objective lens through the second beam splitter and the return light being guided to the common light receiving element through the first and second beam splitters, wherein the first beam splitter has a reflectance set within approximately 70% to 90% for the first laser beam and is set to a full transmittance for the second laser beam, and the second beam splitter is set to a full transmittance for the first laser beam and has a reflectance set within approximately 20% to 60% for the second laser beam.
In the optical pick-up device according to the invention, the first laser beam is reflected by the first beam splitter having a high reflectance with respect to the laser beam and is then guided to the optical recording medium side. Accordingly, the quantity of light of the first laser beam irradiated on the optical recording medium can be maintained sufficiently. Accordingly, it is possible to implement a reliable recording operation by using the first laser beam for recording on the optical recording medium.
For the first and second beam splitters, it is desirable that the first laser beam should be set to be one of P and S polarized lights and the second laser beam should be set to be a polarized light orthogonal to the first laser beam or to include both P and S polarization components for the first and second beam splitters.
If a direction of polarization of the laser beam is thus set, the first and second beam splitters having the optical characteristic can be implemented inexpensively. In general, it is preferable that the second laser beam should be set to include the P polarization component set within approximately 30% to 70% and the S polarization component set within the remaining part.
If the direction of polarization is thus set, an elliptical light spot of the first laser beam and an elliptical light spot of the second laser beam are formed on the optical recording medium with an inclination of approximately 45 degrees. Accordingly, both of the light spots can be formed to have an arrangement relationship suitable for recording and reproduction in a direction of a tangent of a recording track of the optical recording medium.
The first laser beam can serve to carry out recording on the optical recording medium and the second laser beam can serve to carry out reproduction of the optical recording medium.
In this case, it is desirable that a direction of a major axis of the elliptical light spot of the first laser beam which is formed on the optical recording medium should be set to be coincident with a radial direction of the optical recording medium. Thus, the elliptical light spot of the first laser beam has a minor axis direction coincident with a recording pit direction. Therefore, resolving power can be enhanced.